A transponder may be a passive one, i.e. performing load modulation of the magnetic field generated by the reader.
A transponder may be an active one. When a transponder is an active one, i.e. using active load modulation (ALM) for transmitting information to the reader, the transponder generates the magnetic field which simulates load modulation of the reading device field performed by a passive transponder.
ALM needs to be used in case signal generated by passive load modulation is not strong enough to be detected by an RFID interrogator device or reader. This is the case when transponder's antenna is small or located in a challenging environment.
All ALM systems share same requirement to actively transmit bursts of ALM carrier frequency, which are identical to the carrier frequency of the reader. This means that each burst of the ALM carrier frequency starts with the same phase difference to the carrier signal emitted by the reader. Unchanged phase also implies that the two frequencies are identical. Maximum phase drift of transponder reply signal comparing to reader carrier signal inside complete transponder reply frame is specified to be 30° in draft amendment to ISO/IEC 14443 standard.
Constant phase difference between the ALM carrier and reader carrier can be achieved for example in the following way.
Once the ALM transmission starts (complete transmission sequence is called frame and comprises series of carrier bursts separated by gaps) a frequency source in the transponder is used to generate ALM carrier frequency. The frequency source is occasionally corrected during the gaps in the frame when actual carrier bursts are not emitted. During these gaps only the reader carrier signal is present (on transponder antenna) so the reference frequency is available to re-adjust the ALM carrier frequency source. Such synchronization between the reader carrier frequency and the ALM carrier frequency is performed within each transmitted frame and is accordingly called there below In-Frame Synchronization (IFS).
An example of a device performing an IFS is disclosed in EP 272 7255 B1.
More precisely a phase locked loop (PLL) with voltage controlled oscillator (VCO), which generates clock signal with frequency equal to interrogator carrier frequency, is used to generate the ALM clock. Outside of transponder reply frame, when carrier clock is constantly available at PLL input, PLL is put in lock mode. In this lock mode, feedback is closed and PLL is locked to digitized reader carrier signal.
In transponder reply frame, when ALM transmission takes place, the PLL is put in hold mode.
In hold mode PLL feedback is opened and the VCO continues to run with frequency which was previously established. The free-running frequency of VCO is defined by charge which is stored in loop filter. Frequency difference between ALM carrier signal generated by the VCO and reader carrier signal results in phase drift of ALM carrier signal comparing to interrogator carrier signal.
This frequency difference is caused by several sources.
First source is the difference of frequency generated by PLL system in which VCO is induced and the input carrier signal at the moment before the PLL is put on hold.
Second source is charge injection of switch which puts PLL in hold mode (charge injected by switch changes the voltage on VCO control pin, which results in change of VCO frequency).
The third effect is leakage on VCO control pin and loop filter (loop filter input is in high impedance state, voltage on this input is defined by stored charge, charge is modified by leakage current of electronic elements).
The phase drift generated by sources of frequency difference mentioned above has to be corrected before it drifts above the specified maximum. This is done by closing PLL feedback loop at appropriate moments inside transponder reply frame, where the ALM transmission does not take place and the clock signal extracted from interrogator carrier signal is restored on PLL input.
However, ALM carrier bursts produce at the transponder antenna, signal oscillations after each ALM carrier bursts generation. And, such oscillations may disturb the synchronization between the reader carrier frequency and the ALM carrier frequency.
A solution to this problem has been disclosed in WO 2015/033870 A1.
More precisely, in this document, the ALM carrier bursts are generated from a subcarrier modulation by a Binary Phase Shift Keying (BPSK) encoding.
The synchronization is performed in the gap between two transmission bursts. However, due to the higher amplitude during the transmission bursts period and the oscillation properties of the antenna, the signal amplitude on the antenna of the transponder decays slowly without application of specific measures. And, without such specific measures, the decay of the signal on the inductive capacity antenna last too long and does not let enough time for resynchronization.
Accordingly, the solution disclosed in WO 2015/033870 A1 to shorten this decay comprises performing a controlled damping of the oscillation by specific damping means which quickly stops the oscillation built by ALM transmission.
However, such a damping system increases complexity of the ALM transponder.